This invention relates generally to multiple-cavity filter sections having a single input/output resonant cavity, and more particularly to a receive/transmit multiple cavity filter having two sections coupled to an antenna by a single input/output cavity.
An RF resonant cavity (or multiple interconnected cavities) can be used to create a RF filter. The filter may either pass a RF signal over a limited frequency range (a bandpass filter) or exclude an RF signal over a limited frequency range (a notch or bandstop filter), depending upon how the resonator is connected to the overall system. A perfect single cavity device would operate at a single, specific frequency (the resonant frequency), however due to material and other considerations all resonant frequency devices operate over a frequency range which encompasses the resonant frequency.
An RF resonant cavity is realized by having a conductive post within an enclosed conductive cavity. The post is connected to the housing at one end and extends towards the top of the housing. The resonant frequency of the cavity is selected by adjusting the length of the post or adjusting the space between the top of the cavity and the post.
In RF communication systems, it is usually desirable to use the same antenna to both transmit (Tx) and receive (Rx) signals. The Tx and Rx signals occupy different frequency ranges, and the antenna is designed to support both frequency ranges with equal signal strength. It is necessary to filter the signals to and from the antenna so that only the Tx signal is passed to the antenna while only the Rx from the antenna is passed to the receiver.
A typical RF communication system employing a common antenna is shown in FIG. 1. The system consists of two filters 11 and 12, one tuned to pass only the Tx frequency range and one tuned to pass only the Rx frequency range. The two filters are combined at the antenna side by a diplexer 13. The diplexer may be either a separate device or integrated into the design of the filters. FIG. 2 is a partial view of a multi-cavity multiple filter for use in an RF communication system in accordance with the prior art. The multi-cavity filter includes first and second sets of multiple resonant cavities 14 and 16, with the cavities in each set coupled to one another through openings 17 and 18, formed respectively therebetween. For clarity, a portion of the walls has been cut away at 19. One set of resonant cavities 14 forms the transmit filter 11, and the other set of resonant cavities 16 forms the receive filter. The transmit and receive filters are fabricated within the same housing. The Tx and Rx signals are combined into a common connector 21. In this case the center conductor 21 of the connector is split and terminated in loops 22 and 23 with loop 23 extending into output cavity 14a of the filter section 14 which serves as the transmit filter, and the loop 23 extending into the input cavity 16a of the filter section 16 which serves as the receive filter.
The loops are physically attached to the housing or inner conductor of the associated filter cavity. The cross-sectional area between the loops and the return path to the outer housing of the connector is adjusted to tune the output of the filter. Further adjustment of the center conductor loop is required to balance the impedance of the two output cavities, one for the Tx path and one for the Rx path.
The process of tuning and balancing the two loops while maintaining the performance of the filter is quite complicated. Adjusting the loop area for one filter to match the impedance of the second filter alters the tuning of the first filter. This requires re-adjusting of the tuning elements, which in turn influences the impedance of the filter. In addition, it must be performed on each individual filter, it is not a process or characteristic that is common for all filters of a given design.
It is an object of the present invention to provide a simple multi-cavity RF filter having transmitting and receiving filter sections with a single input/output cavity.
There is provided a two-section multi-cavity filter in which the output cavity at one filter section and the input cavity of the other filter section are coupled by a single output/input cavity.